The contractor shall provide and maintain the following: 15 Bovine Leukemia Virus (BLV) infected high-risk cows, 5 BLV-infected low-risk cows, 10 uninfected control cows, 10 BLV-infected sheep and 10 uninfected control sheep. The contractor shall supply from these animals the following materials to outside investigators on a Resource Payback System: Blood, serum, plasma, bone marrow, leukocytes, bone marrow smears and virus-producing leukocytes. Additionally, the contractor shall perform clinical surveillance on the animals, on a regular basis.